How to procedurally generate a veiny skin material?
You can generate a skin material with veins like this using wave textures, musgrave texture, and translucent materials. The waves are distorted to get the meandering of veins. Different wave textures are combined with different orientations to get the veins running in different directions. The musgrave textures is used to make the veins appear and disappear in different areas.

The bright spot is from a lamp below the 'skin' illustrating the translucency.





This material is generated as shown below:





A good reference for generating veins in skin is at https://gooseberry.blender.org/shading-kenny-the-caterpillar/ 



The blend file: